Environmental concerns have led to decreases in the permissible levels of sulfur in hydrocarbon fuels. While reduction in the maximum sulfur level of road diesel oils from about 0.3 weight percent to 0.05 weight percent were implemented in the 1990s, further significant reductions have since come into effect. In the European Union, the Euro IV standard specifying a maximum of 50 wppm (0.005%) of sulfur in diesel fuel for most highway vehicles has applied since 2005; ultra-low sulfur diesel with a maximum of 10 wppm of sulfur was required to be available from 2005 and was, in fact, widely available in 2008. A final target is the 2009 Euro V fuel standard for the final reduction of sulfur to 10 wppm, which is also expected for most non-highway applications.
In the United States, the Environmental Protection Administration has required most on-highway diesel fuel sold at retail locations in the United States to conform to the Ultra Low Sulfur Diesel (ULSD) standard of 15 wppm since 2006 except for rural Alaska which will transition all diesel to ULSD in 2010. Non-road diesel fuel, required to conform to 500 wppm sulfur in 2007, will be further limited to ULSD in 2010 and railroad locomotive and marine diesel fuel will also change to ULSD in 2012. After Dec. 1, 2014 all highway, non-road, locomotive and marine diesel fuel produced and imported will be ULSD.
The allowable sulfur content for ULSD in the United States (15 wppm) is much lower than the previous U.S. on-highway standard for low sulfur diesel (LSD, 500 wppm). The reduced sulfur content not only reduces emissions of sulfur compounds but also allows advanced emission control systems to be fitted that would otherwise be poisoned by these compounds. These systems can greatly reduce emissions of oxides of nitrogen and particulate matter and according to EPA estimates, emissions of nitrogen oxide will be reduced by 2.3 million metric tonnes (2.6 million short tons) each year and soot or particulate matter will be reduced by 100,000 metric tonnes (110,000 short tons) a year with the adoption of the new standards.
In order to meet these regulations refiners currently use costly high pressure hydrogenative processing to desulfurize the hydrocarbons in the fractions used in road diesel oil, much of which comes from a fluid catalytic cracking (FCC) unit, mainly in the form of the light catalytic cycle oil (LCCO) fraction. Unfortunately, nitrogen compounds in this fraction tend to poison the hydrotreating catalysts and for this reason, refiners may undercut the cycle oil and send the higher boiling fractions with problematic sulfur and nitrogen compounds to the heating oil pool at a significant economic loss. Preliminary estimates suggest that elimination of nitrogen compounds in LCCO could be worthwhile as a way of uplifting the higher boiling LCCO components from the heating oil product to road diesel fuel.
Hydrotreatment of the FCC feed to remove sulfur and nitrogen presents a potential solution with a secondary benefit that basic nitrogen compounds which are known to occupy the active cracking sites of an FCC catalyst would be removed, so enhancing the cracking process and increasing the FCC conversion in addition to increasing the processability of the LCCO in subsequent hydroprocessing. Although FCC feed hydrotreaters are in use with low quality feeds, the volumes of liquid to be processed are large and the units themselves are expensive both in capital and operating costs; for this reason treatment of the FCC feed is not a favored option, at least from the viewpoint of refining economics.
A more economically attractive option would be to remove the problematic nitrogen compounds either from the FCC feed or from the cracked products with the advantage in the second case that a smaller volume of liquid would need to be treated. Following treatment to remove the nitrogen compounds, the distillate stream and/or LCCO could be sent to the hydrotreater for desulfurization.
Acid treatment of FCC naphthas has been proposed in U.S. Pat. No. 7,288,181 for the removal of basic nitrogen compounds, using solid acids such as cation exchange resins and zeolites as well as acids such as sulfuric acid. The acid treatment process is less applicable to the treatment of the nitrogen compounds found in cracked distillates since these higher boiling compounds are generally heterocyclic in nature with the nitrogen atom located in an aromatic ring system in which delocalization reduces the basicity of the nitrogen and its reactivity to acids. The removal of heteroatom-containing impurities from kerogen by extraction using a polar solvent system such as water with formaldehyde is described in U.S. Pat. No. 6,875,341.